The invention relates to voltage regulator circuits of the kind commonly called band gap voltage references, and more particularly to improved band gap voltage reference circuits which have high open loop gain, low sensitivity to variations in load current, and are adjustable to scaled-up amplitudes.
The known band gap voltage reference circuits have various shortcomings. Most of them are quite complex, when implemented in integrated circuits, and occupy a large amount of semiconductor die area. Some of the prior band gap voltage reference circuits do not have adequate voltage gain and are unduly sensitive to variations in "load current" which must be supplied to a load circuit by the band gap voltage reference circuit. Some of the prior band gap voltage reference circuits are capable of generating only a particular reference voltage, and cannot be adjusted to produce a higher scaled-up temperature-independent reference voltage.
The closest prior art known to applicants is a band gap voltage reference circuit developed by co-inventor Henry, which utilizes the same "gain cell" or "band gap cell" as the present invention, and provides positive feedback from the output of the circuit to the gain cell. The positive feedback includes an NPN emitter follower output transistor, and an NPN transistor having its emitter connected to the base of the emitter follower output transistor, its collector connected to a current mirror which provides the bias current of the gain cell, and its base connected to the emitters of the PNP transistors that constitute the load devices of the NPN transistors that constitute a differential input pair of the band gap cell. The emitter follower output transistor causes the input offset voltage of the NPN differential input transistor pair of the band gap cell to be developed across a first resistor. A second resistor is connected in series with the first resistor, and the ratio of the first and second resistors is adjusted so that the positive temperature coefficient of the voltage developed across the first resistor offsets the negative temperature coefficient of a diode connected in series with them. The impedance seen at the emitter of the NPN output transistor of this band gap reference voltage circuit is very low, being essentially equal to the sum of the first and second resistors. The bias current of the band gap cell is established by a current which is temperature dependent. This leads to variations with temperature in the input offset voltage of the gain cell, and hence, in the reference voltage produced by this band gap voltage reference circuit. The low input impedance prevents effective scaling up of the band gap voltage produced by this circuit.
In short, there remains a need for an improved band gap reference voltage circuit which is not unduly complex, which can be easily implemented with conventional integrated circuit processing, which has high output impedance, high gain, and has a temperature independent output voltage that is scaled up from the band gap voltage generated from the input offset voltage of the differential pair of the band gap cell, and which is much more independent of variations in load current than previous band gap voltage reference circuits.